Insulation systems with multiple layers of reflective surfaces or films, such as foils and fibrous insulation between the reflective surfaces are known in the art. The use of low emissivity films on the boundaries of an insulation product reduces the thermal conductivity due to radiation. Typically, insulation products of the prior art using reflective films require vacuums. The vacuum requirement gives the insulation product inherent physical limitations, and such a product is unsuitable for use as a basic building material. The loss of the vacuum during transportation, installation and use is high.
Typical building insulation having a density of about 0.6 pounds per cubic foot (pcf) has enough fiber surfaces within the product to constitute a good pathway for absorbing radiative heat transfer. Therefore, typical insulation products of fibrous glass and the like have a significant heat transfer by radiation (k.sub.rad). Typically, the k.sub.rad for 0.6 pcf building insulation product of fibrous glass is on the order of about 0.13 BTU-in./hr Ft.sup.2 .degree.F. A reduction in the density of the insulation product is advantageous in that it reduces freight costs and is potentially easier to install. Further, density reductions will result in lower manufacturing costs due to reduced use of raw materials and energy for the same insulating value.
There is a need for an insulation product which uses low density materials and also makes use of the low radiation heat transfer characteristics of low emissivity surfaces. Such an insulation product would have a high insulation value per unit weight.